The main problems encountered when using fluids under HP/HT (high pressure/high temperature) conditions are essentially due to the following two constraints:                a well fluid density above 2000 kg/m3 is required in order to balance the bottomhole pressure at great well depths,        the bottomhole static temperatures are often much higher than 200° C. and they can sometimes reach or even exceed 300° C.        
The drilling techniques commonly used under less drastic conditions (less deep reservoir and/or temperature less than or equal to 200° C.) utilize water-base fluids or oil-base fluids. In the latter case, the continuous phase generally consists of a hydrocarbon of diesel or isoparaffin type. The density of the fluid is then adjusted by adding dense salts soluble in aqueous phases (barium chloride, alkaline formates, . . . ), and also by adding mineral fillers dispersed in the continuous phase. The dispersion stability is generally provided by various types of suspending, thinning, wetting agents, polymers.
The fluids currently known are ill-suited to HP/HT conditions, in particular above 220° C. The main problem of current muds under HP/HT conditions is thus when drilling is stopped. After an extended period of time in the well bottom, a strongly marked particle settling phenomenon, notably weighting material, appears. In some cases, a solid mass that cannot be recirculated forms. The well is then plugged.
This phenomenon is linked to the high mineral filler concentration in the fluid formulation, which is necessary to densify the mud, and to the settling acceleration due to the loss of the rheological properties of the fluid. The loss of Theological properties is notably due to the thermal degradation of the various additives the mud is made up of, notably the suspending agents (such as clays, polymers, surfactants). As regards hardening of the deposit, gelling of the clays under the effect of temperature is probably involved.
For water-base fluids, hydrosoluble polymers are used as viscosifiers for the continuous phase. They are of polysaccharide type, such as xanthan derivatives or others, or synthetic polymers, vinyl copolymers or terpolymers, based on acrylamide and comprising at least one of the following monomer units: acrylate, acrylamidomethyl propane sulfonate (AMPS), itaconate, styrene sulfonate. The limit temperature of use for this type of polymer rarely exceeds 200° C. The following documents can be mentioned by way of reference: Y. M. Wu, B. Q. Zhang, T. Wu, C. G. Zhang: Colloid Polym. Science 27 (9) 836-84 (2001); A. Audibert, L. Rousseau, J. Kieffer, SPE 50724, (1999); Y. M. Wu, D. J. Sun, B. Q. Zhang, CH. G. Zhang: J. Appl. Polym. Sci., 83, 3068-3075 (2002).
The mineral fillers can also be suspended by adding to the formulation dispersing polymers that are also limited to a similar temperature range.
In the case of non aqueous fluids, notably fluids comprising isoparaffins as the continuous phase, the formulation is more complex. They are reverse water-in-oil emulsions whose continuous phase density is about 900 kg/m3. When the density of the fluid is necessarily high (density greater than or equal to 2000 kg/m3), the volume fraction of fillers incorporated in the fluid can exceed 35%. The stability of these formulations is linked with the stability of the emulsions at high temperature and, then again, it is very difficult to formulate stable fluids at temperatures above 220° C. The risk involved is then a phase separation that causes degradation of the rheological properties, encroachment of the formations and settling of the weighting agents (P.D. Scott, M. Zamora, C. Aldea, IADC/SPE Drilling Conference, 2-4 March, Dallas, Tex.—D. J. Oakley, et al. IADC/SPE Asia Pacific Drilling Technology, 11-13 September, Kuala Lumpur, Malaysia). The deposits can be plentiful and gel after some ten hours under the effect of temperature.
The present invention relates to well fluids comprising dense liquids stable at high temperatures, which are the main constituents of the continuous phase. The invention more particularly relates to well fluids whose continuous phase comprises a fluorinated dense liquid stable at high temperature. The preferred range of application of these fluids is generally under high pressure and high temperature conditions generally encountered during drilling or development of very deep-seated geologic reservoirs, i.e. more than 3000 m, generally more than 5000 m in depth.